Certain organic substances, known as sunscreens, whose molecules absorb the harmful ultraviolet rays are widely used in cosmetic compositions to protect human skin against the damaging effects of sunlight. Sunscreens are described in many publications, including Nicoll et al., U.S. Pat. No. 5,188,831 and Dobrowski et al., U.S. Pat. No. 5,961,961. The effectiveness of sunscreens is measured in terms of a property known as the Sun Protection Factor (SPF), as is well know to one skilled in the art.
The ideal sunscreen should be sufficiently chemically and physically stable so as to provide an acceptable shelf life upon storage. It is particularly desirable that the preparation should retain its protective effect over a prolonged period after application. Thus, the active agent when present on the skin must be resistant to chemical or photodegradation, to absorption through the skin, and to removal by perspiration, skin oil, or water.
Sunscreen agents in the order of decreasing effectiveness may be categorized as either highly chromophoric monomeric organic compounds, inorganic compounds and minimally chromophoric polymeric organic solids.
U.S. Pat. No. 5,219,558 (Woodin, Jr. et al.) and U.S. Pat. No. 4,919,934 (Deckner et al.) disclose photoprotection compositions wherein the active sunscreen agents are of the chromophoric monomeric organic compound variety. The examples feature the commercially common sunscreens such as octyl methoxycinnamate (Parsol MCX), benzophenone-3(Oxybenzone) and octyl dimethyl PABA.
Chromophoric monomeric, organic compounds are subject to certain problems. Organic sunscreens, when incorporated in personal care compositions or when deposited on skin, lose their sunscreen properties and may change character with time, due to many factors.
Among many reasons for degradation, one factor responsible for loss of sun screen factor (SPF) or change in character is that organic sunscreens are oxidatively, or photooxidatively, unstable. Instability is an undesirable characteristic in organic sunscreens. There is a need, therefore, for an agent that will stabilize organic sunscreens against degradation. In particular, there is a need for an agent that will prevent the oxidation or photooxidation of organic sunscreens.